The US Environmental Protection Agency is demanding the trucking industry reduce its emissions by 90% in 2007. It's no small feat, and in order to comply, manufacturers will have to introduce new comp...
DPF CLEANER: Cleaning machines such as this one by Caterpillar are required to clear the DPF of ash, which accumulates during use.
INSIDE LOOK: The filter inside this Cummins DPF traps soot, which is then burned off through a process called regeneration.
The US Environmental Protection Agency is demanding the trucking industry reduce its emissions by 90% in 2007. It’s no small feat, and in order to comply, manufacturers will have to introduce new components as well as a costly aftertreatment system. Between now and December, Truck News will be exploring, in detail, each element of the 2007 emissions equation. We continue the series this month with Part 3 – a look at diesel particulate filters.
TORONTO, Ont. – When you first lay eyes on a 2007 truck, the most obvious change will be the addition of a diesel particulate filter (DPF). It’s a cylindrical can attached to the bottom of the truck frame or the rear of the cab, taking the place of a traditional muffler.
With the last round of emissions standards, introduced in 2002, NOx was the primary target. This time around, it’s particulate matter. The DPF’s job is to trap particulate matter such as soot, and burn it off at extremely high temperatures. The residual matter is ash, which must periodically be cleaned from the filter.
The DPF consists of an oxidation catalyst comprised of precious metals. These metals make the DPF a pricey piece of equipment – much of the US$7,500-$10,000 cost increase associated with 2007 trucks can be attributed to the particulate filter.
According to Cummins, “A typical filter consists of an array of small channels that the exhaust gas flows through. Adjacent channels are plugged at opposite ends, forcing the exhaust gas to flow through the porous wall, capturing the soot particles on the surface and inside pores of the media. Soot accumulates in the filter, and when sufficient heat is present a ‘regeneration’ event occurs, oxidizing the soot and cleaning the filter.”
There are two types of regeneration: Passive and Active.
Passive regeneration will occur most of the time and will require no input from the driver. As the truck is rolling down the highway, the temperatures required for a regeneration to take place (about 300 C) will usually be reached.
When the filter requires cleaning and the exhaust is hot enough to perform a passive regeneration, it will take place without the driver even realizing it.
“There’s no light, no change in audible noise, no change in engine speed,” says Dr. Steve Charlton, executive director, heavy-duty engineering with Cummins. “It’s totally transparent to the operator.”
Occasionally, in some duty cycles, a truck’s engine will not reach the temperatures required for passive regeneration to occur. This will generally be the case when hauling light loads, operating in stop-and-go traffic or driving in the dead of a Canadian winter.
When the exhaust doesn’t reach the temperatures required for a passive regeneration, an active regeneration will be required. With most DPFs, a fuel dosing system is the answer. An injector shoots a small amount of diesel fuel into the filter, sparking an increase in exhaust temperatures, facilitating a regeneration.
The amount of fuel required to spark an active regeneration is subject for debate. Some reports have it at up to half a gallon of fuel. Cummins’ Charlton suggests it would be much less.
“We use a rather small injector so a very small fuel quantity is used,” he says.
For its part, Caterpillar has designed its own DPF which doesn’t involve a dosing system at all. Instead, it utilizes a Cat Regeneration System (CRS) that works much like a gas-fired furnace. The fuel is injected into a closed combustion chamber rather than being dosed into the DPF itself.
“We wanted a product that would be able to regenerate under all conditions,” explains Greg Gauger, director, Caterpillar On-Highway Power Systems. “We designed a process to allow our product to accomplish this safely, without the risks created by dosing.”
The risks of dosing have been addressed by the OEMs, by only allowing active regeneration to take place when the truck is in motion, eliminating concerns of nearby trees or branches catching fire.
“At no time will an active regeneration automatically take place with the vehicle parked,” emphasizes David McKenna, product marketing manager for engines, transmissions and axles with Mack Trucks.
In rare cases under some duty cycles a driver may have to manually trigger an active regeneration.
“This would be something as difficult as activating a dash-mounted switch,” McKenna says.
Mounting options for the DPFs vary, depending on application. For highway trucks, the DPF will usually be mounted on the frame rails underneath the cab. A vertical back of cab mounting – similar to today’s smokestacks – will also be seen.
The higher horsepower Caterpillar engines, such as the C15 550 hp powerplant, will require two DPFs.
Eventually, all DPFs will require cleaning. The US Environmental Protection Agency requires that all DPFs be able to run 150,000 miles between service intervals. By most accounts, it will be possible to substantially extend that cleaning interval.
“We expect a minimum of 4,500 engine hours until the first DPF ash removal service,” says Mack’s McKenna. “This compares with approximately 150,000 engine miles for typical highway operations.”
But other manufacturers have expressed more optimism.
Tim Tindall oversees Detroit Diesel’s 2007 engine program. So far in field testing he says DPFs with more than 100,000 miles on them are showing no signs of clogging.
“We’ve looked at a number of filters with well over 100,000 miles on them and we don’t see any ash on them,” Tindall says. “That’s a pleasant finding – the ability to store the ash and distribute it evenly throughout the filter so we can get high maintenance intervals on the ash cleaning.”
He even suggests it may be possible to run 500,000 miles before cleaning.
Under favourable operating conditions, Charlton says Cummins DPFs are able to run a minimum of 200,000 miles before requiring cleaning “and it could be as high as 300,000-400,000 miles.”
In mid-September, the highest mileage field test unit in real-world testing, had racked up 175,000 miles with no signs of ash buildup, Charlton tells Truck News.
Cat officials say their DPFs will be able to run 200,000-300,000 miles between cleanings.
But alas, all DPFs will eventually need to be cleaned. It’s expected there will be two options: an exchange program where customers can swap their dirty filter for a clean one; or a quick on-site cleaning using a special DPF cleaning machine.
Caterpillar has developed a tool allowing its DPFs to be cleaned while attached to the truck. The company says it takes about as long as an oil change to clean the filter.
Most other filters will have to be removed and placed on the cleaning machine, which operates like your typical shop air pressure system. Typically, only a few grams of ash will have accumulated in the filter between cleanings.
The entire process – including removing and re-attaching the DPF – takes about an hour and can be completed during a regularly-scheduled oil change.